Dendritic cells comprise a system of highly efficient antigen-presenting cells which initiate immune responses such as the sensitization of T cells restricted by major histocompatibility complex molecules, the rejection of organ transplants and the formation of T-cell-dependent antibodies. Dendritic cells are found in many non-lymphoid tissues, such as skin (Langerhans cells) and mucosa, and they migrate after antigen capture through the afferent lymph or the bloodstream to lymphoid organs, where they efficiently present antigen to T cells. Dendritic cells are difficult to isolate and, although they originate from bone marrow their site of maturation and the conditions that direct their growth and differentiation are still poorly characterized. Granulocyte macrophage-colony stimulating factor (GM-CSF) favours the outgrowth of dendritic cells from mouse peripheral blood. Here we extend this finding to man and demonstrate that cooperation between GM-CSF and tumour necrosis factor-alpha (TNF-alpha) is crucial for the generation of human dendritic/Langerhans cells from CD34+ haematopoietic progenitors. The availability of large numbers of these cells should now facilitate the understanding of their role in immunological regulation and disorder.

SummaryHuman dendritic cells (DC) can now be generated in vitro in large numbers by culturing CD34 + hematopoietic progenitors in presence of GM-CSF+TNFet for 12 d. The present study demonstrates that cord blood CD34 + HPC indeed differentiate along two independent DC pathways. At early time points (day 5-7) during the culture, two subsets of DC precursors identified by the exclusive expression of CDla and CD14 emerge independently. Both precursor subsets mature at day 12-14 into DC with typical morphology and phenotype (CDS0, CD83, CD86, CD58, high HLA class II). CDla + precursors give rise to cells characterized by the expression of Birbeck granules, the Lag antigen and E-cadherin, three markers specifically expressed on Langerhans cells in the epidermis. In contrast, the CD14 + progenitors mature into CDla + DC lacking Birbeck granules, E-cadherin, and Lag antigen but expressing CD2, CD9, CD68, and the coagulation factor XIlla described in dermal dendritic cells. The two mature DC were equally potent in stimulating allogeneic CD45RA + naive T cells. Interestingly, the CD14 + precursors, but not the CDla + precursors, represent bipotent cells that can be induced to differentiate, in response to M-CSF, into macrophage-like cells, lacking accessory function for T ceils.Altogether, these results demonstrate that different pathways of DC development exist: the Langerhans cells and the CD14+-derived DC related to dermal DC or circulating blood DC. The physiological relevance of these two pathways of DC development is discussed with regard to their potential in vivo counterparts.

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